During the Ice Ages water was locked up in glaciers, polar sea ice and ice sheets. Sea levels dropped (120m) allowing migration of people and animals. In the UK, temps were reduced by 15/20ºC. Just 600 generations ago the northern hemisphere was in full glaciation with a third of all land covered by ice and 5% of the world’s oceans frozen. The switch from cold to warm and vice versa can occur rapidly – within decades – and the disruption of ocean currents can have serious and far-reaching consequences for climatic change.
Why we have glacials and interglacials Long Term Forces 3 Milankovitch Cycles Eg the Cryogenian (after cryogen for freezing mixture) between 800 and 600 million years ago, the Sun was weaker and the Earth received 6% less insolation. Also, greenhouse gases (CO² and methane) were insufficient to hold back the bitter cold of space. Ice sheets 1km thick engulfed the world from the poles. This would have increased the world’s albedo affect reducing temps to about – 50ºC. Short Term Forces Sun Spot Activity Volcanic Eruptions – screening out ISR Deforestation - +CO2 Positive feedback – albedo effect NADW formation
So what is the long and short term evidence for glacial cycles Forams – small sea creatures Ice Cores – Eg Greenland Deep Ocean Cores Tree rings Coral Erratics Landforms formed by glaciers – their imprints left Species migration and Fossil record Photos and writings Tax records farm land – eg Switzerland
One theory that has been put forward to explain the Ice Ages is: Milankovich cycles It considers both changes in the tilt of the Earth’s axis and variations in its orbit around the Sun. The Earth tilts on its axis, if it didn’t we wouldn’t have seasons. The tilt averages about 23.5 degrees, but it is not constant. The Earth wobbles or precesses about its axis over 23/26,000 years. This causes the tilt to change between 22 and 25 degrees over 41,000 years. MILANKOVITCH THEORY
Milankovich cycles – variations in shape of Earth’s orbit around Sun
Deep Water Formation It is said that GW could delay an Ice Age but it could also bring its return (at least as far as the UK and Northern Europe are concerned). Why is it possible to have tropical palms in western Ireland and SW England? = GULF STREAM One of the ways to weaken or shut down the gulf stream would be to inject large quantities of cold, fresh water into the N Atlantic. If GH gas emissions double over the next 70 years then the warming currents could weaken by 30%. Under a ‘business as usual’ scenario, GH gas emissions will quadruple by 2100. This will lead to a complete shutdown by the middle of the next century.
Little Ice Age 1450 (possibly 1200) – 1850 (and start of 20 th C.) glaciers advanced. Engulfing alpine villages Fishing industries disrupted Eskimos south paddled to Scotland Thriving Viking community in Greenland was cut off and never heard from again. Annual mean temps in England (late 17 th C.) were almost 1 degree lower than 1920-60. Even in 1800’s people ice skated on the Thames regularly in winter NB: Medieval Warm Period – 1000 – 1300AD.
The Great Global Warming Experiment Earth is now warmer than it has been for over 90% of its 4.6 billion year history. By the end of the 21 st century our planet may see higher temps than at any other time for the last 150,000 years. This century the decade of the 1990’s has been the hottest (all across the planet as a whole). These are some of the ways in which the climate can be affected: Variations is the Suns output – Sunspot Cycle Gas emissions Volcanoes Nuclear testing
Hothouse Earth By 2100, global temps are forecast to rise by up to 8ºC over land, with sea levels up to 88cm higher. CO² conc. in the atmosphere will be higher than at any time in the last 20 million years. By 2025, 5 billion people will live in countries with inadequate water supplies In 50 years all the world’s great reefs will have gone due to high sea temps. No more winter sports in 2100 If the Greenland Ice Sheet melts, coastal cities will be drowned (NYC, London, Sydney).